An image display apparatus such as the PDP or the DMD, which performs a binary control of light emission and non-light emission, typically uses a method in which multi-level gradations are displayed by dividing an image of one field into images of a plurality of subfields, so called a subfield method. In the subfield method, a period of one field is divided into a plurality of subfields weighted with the number of light emission or the amount of light emission by temporal decomposition, the display of a gradation level is performed by the combination of subfields to be emitted.
FIG. 13 is a schematic view showing an example of configuration of a subfield in the PDP. In this example shown in FIG. 13, a single field is divided into eight subfields (SF1, SF2, . . . , and SF8), wherein respective subfields have luminance weights (1, 2, 4, 8, 16, 32, 64, and 128). Each subfield is composed of setup period (T1) for preliminary discharge, writing period (T2) during which data for light emission or non-emission is written for each pixel, and sustain period (T3) during which pixels with light-emitting data being written are made to emit light all at once. Combining these subfields for emitting light can produces 256 gradation levels of “0” level through “255” level. For example, gradation level “7” is presented by emitting SF1, SF2, and SF3 having luminance weights 1, 2, and 4, respectively, and a gradation level “21” is presented by emitting SF1, SF3, and SF5 having luminance weights 1, 4, and 16, respectively.
In such a display method that uses the subfield method for displaying multi-level gradation, it is known that a case where the deterioration of image quality is observed while motion pictures are displayed occurs. False contours (dynamic false contours) are one of the reasons for the deterioration of image quality. The dynamic false contours are described hereinafter for an exemplary case where a single field is divided into eight subfields (SF1, SF2, . . . , and SF8) respectively luminance-weighted with (1, 2, 4, 8, 16, 32, 64, and 128).
FIG. 14 is a schematic view showing an example of an image pattern moving on the screen of the PDP horizontally. The image pattern shown in FIG. 14 has region P1 with gradation level “127” and region P2 with gradation level “128”, and moves horizontally to the left-to-right direction or the right-to-left direction on the screen of the PDP. FIG. 15 is a schematic diagram in which the image pattern shown in FIG. 14 is expanded to subfields. In FIG. 15, a horizontal axis corresponds to a horizontal position on the screen of the PDP, and a vertical axis corresponds to a time direction. In addition, hatched subfields represent non-emission subfields.
When the image pattern remains stationary, and a viewer's viewpoint is also fixed to screen position A rather than moving in a horizontal direction, as shown in an arrow A-A′ of FIG. 15, the viewer can perceive region P1 and region P2 as regions having the gradation levels “127” and “128,” respectively, which are original gradation levels.
However, when the image pattern moves in a right-to-left direction on the screen of the PDP and the viewer follows the movement of the image pattern to move the viewpoint to the direction of arrow B-B′, there is a case where the non light-emitting subfields in region P2 (SF1 to SF7 in region P2) and the non light-emitting subfields in region P1 (SF8 in region P1) are viewed as a continuous pattern by the viewer. In the case, subfields SF1 to SF8 are perceived as continuous non light-emitting subfields by the viewer, and consequently, gradation level “0”, that is, a dark line is perceived.
To the contrary, when the image pattern moves in a left-to-right direction on the screen of the PDP and the viewer follows the movement of the image pattern to move the viewpoint in a direction of an arrow C-C′, there is a case where the light-emitting subfields in region P1 (SF1 to SF7 in region P1) and the light-emitting subfields in region P2 (SF8 in region P2) are viewed as a continuous pattern by the viewer. In the case, subfields SF1 to SF8 are perceived as continuous light-emitting subfields by the viewer, and consequently, gradation level “255”, that is, a bright line is viewed. In any of the cases described above, a region of gradation level which is considerably different from the original gradation levels “127” and “128” is viewed as if the region exists on the screen of the PDP. The region is a false contour which occurs during displaying a motion picture, that is, a dynamic false contour, and the dynamic false contour is one of the reasons for deterioration of image quality.
As is clear from the principle of occurrence of the dynamic false contour described with reference to FIG. 15, dynamic contours may easily occur in a case where the pattern of light-emitting subfields markedly changes in spite of small change in gradation levels. For example, when weighted subfields as described above are used, the false contour may be easily observed in a case where the luminance gradation levels of the adjacent pixels are “63” (SF1 to SF6 are light-emitting subfields) and “64” (SF7 is a light-emitting subfield), “191” (SF1 to SF6 and SF8 are light-emitting subfields) and “192” (SF7 and SF8 are light emitting subfields), or the like, since the pattern of light-emitting subfields markedly changes in spite of small change in gradation levels.
As a technology for suppressing the dynamic false contours, a method of converting a gradation level of a video signal to be displayed into a gradation level for which the dynamic false contour is difficult to occur is proposed. In this method, at first, a motion picture region of an image (hereinafter, an image region in which movement occurs between frames or fields is simply referred to a motion picture region, and an image region in which movement does not occur between frames or fields is simply referred to as a still image region) is detected by calculating a difference between frames or fields of a video signal or the like. For a region determined to be a still image region, an original gradation level is used for display (hereinafter, a process of a video signal in the still image region is simply referred to as a still image process), and for a region determined to be a motion picture region, an original gradation level is converted into a value for which the dynamic false contour is difficult to occur for display (hereinafter, a process of a video signal in the motion picture region is simply referred to as a motion picture process).
FIG. 16 is a table showing an example of gradation levels for which the dynamic false contour is difficult to occur. For example, when a single field is divided into eight subfields (SF1, SF2, . . . , and SF8) having luminance weights of (1, 2, 4, 8, 16, 32, 64, and 128), gradation levels (hereinafter, simply referred to as gradation levels for motion picture display) for which the dynamic false contour is difficult to occur are “0”, “1” (light-emitting subfield is SF1), “3” (light-emitting subfields are SF1 and SF2), “7” (light-emitting subfields are SF1 to SF3), “15” (light-emitting subfields are SF1 to SF4), “31” (light-emitting subfields are SF1 to SF5), “63” (light-emitting subfields are SF1 to SF6), “127” (light-emitting subfields are SF1 to SF7), and “255” (light-emitting subfields are SF1 to SF8). As in a case where the light-emitting subfield is “0”, or the gradation levels are “1”, “3”, “7”, “15”, “31”, “63”, “127”, and “255”, when light-emitting subfields display an image using successive gradation levels from a subfield which has a least luminance weight, a change in the pattern of the light-emitting subfields between adjacent pixels can be suppressed to be small, and accordingly the occurrence of the dynamic false contour can be suppressed.
However, in this method, when compared with a case where a still image region is displayed with 256 gradation levels of “0” level to “255” level, there are only nine motion picture gradation levels of “0”, “1”, “3”, “7”, “15”, “31”, “63”, “127”, “255” which can be used for a motion picture region. Therefore, a method of calculating an error generated in converting an original gradation level into a gradation level for motion picture display and diffusing the error to neighbor pixels, so called error diffusion is used simultaneously. By using the error diffusion, a difference between a gradation level used for display and an original gradation level is interpolated, and accordingly, the small number of gradation levels in the motion picture region can be supplemented.
FIG. 17 is a schematic diagram showing an example of the error diffusion. For example, when a i-th pixel of M-th line is converted into a gradation level for motion picture display (M and i are natural numbers), for example, when the original gradation level is “95”, a gradation level for motion picture display which is the closest to “95” is “127”, and accordingly, the gradation level of the M-th line and i-th pixel is converted into “127”. At this time, since an error of “127−95=32” occurs, the error “32” is diffused to neighbor pixels. To be more specifically, a value equals to 7/16 times “32”, that is, “32× 7/16=14” is added to a gradation level of the adjacent (i+1)-th pixel of M-th line. Likewise, a value equals to 3/16 times “32”, that is, “32× 3/16=6” is added to a gradation level of the adjacent (i−1)-th pixel of (M+1)-th line, a value equals to 5/16 times “32”, that is, “32× 5/16=10” is added to a gradation level of the adjacent (M+1)-th line and i-th pixel, and a value equals to 1/16 times “32”, that is, “32× 1/16=2” is added to a gradation level of the adjacent (i+1)-th pixel of (M+1)-th line.
In addition, for a pixel to which the diffused error is added, a gradation level for motion picture display which is the closest to a result from the addition of a diffused error to the original gradation level is selected for a gradation level used for display, and the error occurs at that time is diffused to neighbor pixels as described above.
As described above, the dynamic false contour is decreased by using the gradation levels for motion picture display for the motion picture region and the reduction of the number of gradation levels for the motion picture region is suppressed by interpolating between a gradation level for display generated and the original gradation level using the error diffusion. The technology described above is disclosed in Japanese Patent Unexamined Publication No. 2000-276100.
However, a boundary between a region in which a motion picture process is performed and a region in which a still image process is performed is formed due to the switch of video signal processing methods between the motion picture region and the still image region, and there is a case where a noise (hereinafter, referred to as switching shock) in the shape of a sharp edge occurs in the boundary. Accordingly, a method of decreasing the switching shock by generating a random number and diffusing the boundary between the motion picture region and the still image region using the random number is, for example, proposed in Japanese Patent Unexamined Publication No. 2003-69922.
In the general technology described above, the switching shock can be decreased by randomly diffusing the boundary between a region in which a motion picture process is performed and a region in which a still image process is performed, using a random number. However, there is a case where a noise in the shape of a sharp edge is left as a dull noise having a fixed width due to randomly diffusing the boundary between a region in which a motion picture process is performed and a region in which a still image process is performed up to the fixed width, and accordingly, the effect of the decrease of the switching shock is not sufficient.